The fire that prompted the evacuation of about 3,400 people from Liberty Island and injured a construction worker Monday was sparked by an accidental “ignition of propane vapors,” the FDNY said Tuesday.

The two-alarm fire started around 11:50 a.m. at a construction site on the island, when three 100-pound propane tanks were accidentally ignited during roofing work, according to a tweet posted on the FDNY Twitter page.

“With that much propane, a [boiling liquid expanding vapor explosion,] which would have been one of the tanks exploding, could have been catastrophic,” the Fire Department’s Manhattan borough commander, Roger Sakowich, said at a news conference Monday.

“Scrap metal would have been flying for quite some distance, plus a tremendous ball of fire when a propane tank does explode like that,” Sakowich said. “So we took the precaution to evacuate.”

One worker suffered minor burns from the flames, but refused medical attention.

The statue reopened about 2:20 p.m. Monday, fire officials said.